Which component's testing requires the engine to be running?

The testing of the compressor clutch voltage measurement requires the engine to be running because the clutch is connected to the engine via a belt drive. When the engine is running, the compressor engages, allowing the system to create the necessary conditions for the clutch to operate. This engagement is critical for accurately measuring the voltage supplied to the clutch, as it needs to be energized to engage properly and activate the compressor. Only under operating conditions can you observe its function and determine whether the voltage is within acceptable parameters.

In contrast, the other options can be tested without having the engine running. The blower motor performance test, for instance, can assess the blower motor's operation independently. The refrigerant pressure check is typically performed with the engine off, allowing for accurate gauge readings on the refrigerant system without the complications of engine influence. Similarly, the radiator temperature assessment can be conducted safely with the engine off or during idle if required, focusing on the coolant's condition without the need for the compressor’s operation.